please help! cannot modify - ITimeSheetEntry

I'm trying to modify ITimeSheetEntry objects in RTC 3.0.1.
code

   IWorkItemWorkingCopyManager wcm = workItemClient.getWorkItemWorkingCopyManager();

                   wcm.connect(handle, IWorkItem.FULL_PROFILE, null);

                    WorkItemWorkingCopy wc = wcm.getWorkingCopy(handle);

                    IWorkItemReferences references = wc.getReferences();
	

                    List<IReference> listReferences = references.getReferences(WorkItemEndPoints.WORK_TIME);

                    ITimeSheetEntry newEntry = null;
	

                    for (IReference reference : listReferences) {
	

                        if (reference.isItemReference()) {
	

                            IItemHandle iReferenceh = ((IItemReference) reference).getReferencedItem();
	

                           if (iReferenceh instanceof ITimeSheetEntryHandle) {

                               ITimeSheetEntry newEntryTemp =  (ITimeSheetEntry) itr.itemManager().fetchCompleteItem(iReferenceh, IItemManager.DEFAULT, null);

                                                                              ....
	

                                                                                ......

                                                                                if(XXXXSOMEXXXX){

                                        hours = new Double(durationHours);

                                        newEntryTemp.setHoursSpent(hours);

                                        IItemReference timesheetRef = IReferenceFactory.INSTANCE.createReferenceToItem(newEntryTemp);

                                        //copy the references

                                        IWorkItemReferences copyReferences = wc.getReferences();

                                        copyReferences.add(WorkItemEndPoints.WORK_TIME, timesheetRef);

                                        IDetailedStatus result = wc.save(null);
                                        break;

                                    }

                                }

                            }

                        }

                    }

                    if (newEntry == null) {
	

                        System.out.println("NEWWW");

                        //create a new time sheet entry

                        newEntry = workItemClient.createTimeSheetEntry(iProjectArea.getProjectArea());

                        int durationHours = Integer.parseInt(duration);

                        newEntry.setHoursSpent(new Double(durationHours));
	
	

the error

com.ibm.team.repository.common.internal.ImmutablePropertyException

    at com.ibm.team.repository.common.internal.util.ItemUtil$ProtectAdapter.notifyChanged(ItemUtil.java:2060)

    at org.eclipse.emf.common.notify.impl.BasicNotifierImpl.eNotify(BasicNotifierImpl.java:280)

    at com.ibm.team.workitem.common.internal.model.impl.TimeSheetEntryImpl.setInternalTimeSpent(TimeSheetEntryImpl.java:553)

    at com.ibm.team.workitem.common.internal.model.impl.TimeSheetEntryImpl.setHoursSpent(TimeSheetEntryImpl.java:961)

    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)